1. Field of the Invention
The present invention relates generally to a method for conducting mobile surveys of road surface, and more specifically to such a method for obtaining continuously image information of cracks, ruts and the like of the road surface from a moving vehicle. The information thus obtained is essentially free of detrimental influences by noise produced by daylight, light emitted from external light sources such as vehicle headlights etc. and therefore is suited for computer based automatic image data processing.
2. Description of the Prior Art
It is known in the art to successively obtain information of cracks, ruts and the likes of road surface while a vehicle is travelling in which a survey apparatus is mounted.
One known technique for detecting cracks and the like fissure type defects of road surface, has utilized an incandescent light or laser beam which is directed to the road surface at right angles to the direction in which a vehicle is travelling. The lights reflected on the road surface is then picked up at a movie or video camera in order to make optical records of the road surface. However, this prior art technique is limited to night use when there is little or no traffic. Further, the optical records are liable to be degraded by various undesired reflective objects such as white lane strips painted on the road surface and contaminated spots left thereon, etc. Accordingly, the optical records are not suited for automatic image data processing using computer.
On the other hand, in order to detect a profile of road such as ruts or wheel tracks, a combination of strobe lights and a pulse camera has been proposed to form "black hair lines" on the road surface. Alternatively, a laser beam is projected onto the road surface to make "bright hair lines" thereon which are recorded by a television camera for example. With these vehicle road surveys, like problems are pointed out that such surveys should be implemented at night. Further, the contrast of the black or bright hair line is readily influenced by the light emitted from the headlights of vehicles in near vicinity of the vehicle in which the survey apparatus is mounted.